1. Field of the Invention
The invention relates to a method for manufacturing sintered oxidic nuclear fuel bodies from uranium oxide starting powder, a mixture of uranium oxide and plutonium oxide starting powder or of uranium-plutonium oxide mixed crystal starting powder. The starting powder may have the composition UO.sub.2+x, PuO.sub.2+x or (U,Pu)O.sub.2+x, in which x typically has a value of 0.1. A U.sub.4 O.sub.9 or (U,Pu).sub.4 O.sub.9 crystal phase is developed in the oxidic nuclear fuel starting material and blanks of such material having the crystal phase developed therein are sintered at a temperature of 1000.degree. to 1400.degree. C. in an oxidizing atmosphere and subsequently in a reducing atmosphere.
2. Description of the Prior Art
European patent application No. 0078428 and related U.S. application Ser. No. 436,303 filed Oct. 25, 1982, now U.S. Pat. No. 4,578,229 disclose a method for manufacture of synthetic fuel involving a U.sub.4 O.sub.9 or (U,Pu).sub.4 O.sub.9 crystal phase. Due to the U.sub.4 O.sub.9 or (U,Pu).sub.4 O.sub.9 crystal phase which is developed in the blanks to a crystallographically detectable extent, the sintered oxidic nuclear fuel bodies obtained according to the known method exhibit a microstructure with uniformly distributed coarse grain, which is stable against growth at the operating temperature in a nuclear reactor. Since grain boundary migration no longer occurs, gaseous or highly volatile nuclear fission products such as xenon or iodine cannot be washed to the outside from the nuclear fuel oxide matrix by migrating grain boundaries and build up an undesirable overpressure in the cladding tube of a fuel rod, in which the oxidic sintered nuclear fuel bodies are located in the nuclear reactor. Furthermore, the blanks need not contain sintering additives which have grain growth promoting action and can influence the density of the sintered nuclear fuel bodies.
The U.sub.4 O.sub.9 or (U,Pu).sub.4 O.sub.9 crystal phase is developed in the known method in the blanks while they are being heated to the sintering temperature in a gas atmosphere with oxidizing action. In order to develop this U.sub.4 O.sub.9 or (U,Pu).sub.4 O.sub.9 crystal phase to a sufficient degree, holding times are as a rule required with the known method, in which the blanks are kept at a temperature in the range of 400.degree. C. to 600.degree. C. when heated up to the sintering temperature in the gas atmosphere with oxidizing action. The length of these holding times depends on the oxygen-to-uranium, i.e. O/U-ratio of the uranium oxide starting powder. A uranium starting powder with a typical O/U-ratio of 2.10 requires, for instance, a holding time of at least 1.5 hours at a temperature in the range of 400.degree. C. to 600.degree. C., if the blanks are to be transformed entirely into the U.sub.4 O.sub.9 or (U,Pu).sub.4 O.sub.9 crystal phase.
The concentration of the uniformly distributed coarse grain in the microstructure of the oxidic sintered nuclear fuel body obtained by the known method depends on the oxygen potential which the gas atmosphere with oxidizing action has during the heating up to the sintering temperature and during sintering.
The oxygen potential is defined by the relation .DELTA.G.sub.O.sbsb.2 =RT.ln p.sub.O.sbsb.2 with R=the general gas constant, T=the absolute temperature and p.sub.O.sbsb.2 =the partial oxygen pressure in the gas atmosphere with oxidizing action.